Fuel cell gas separator plate

ABSTRACT

A fuel cell gas separator ( 212 ) for use between two solid oxide fuel cells ( 210 ) and having a separator body with an anode-facing side and a cathode-facing side and with paths ( 234 ) of electrically conductive material therethrough in an electrode-contacting zone ( 236 ). In a first aspect, the electrically conductive material comprises a silver-glass composite, preferably containing 15 to 30 wt % glass. In this aspect the material of the separator body is preferably zirconia and the silver is commercially pure, a silver mixture or a silver alloy. In another aspect, the material of the separator body is zirconia, the electrically conductive material comprises silver or a silver-based material, a coating of nickel is formed on the electrode-contacting zone ( 236 ) on the anode-facing side preferably with an undercoating of Ag, and a coating of Ag—Sn alloy is formed on the electrode-contacting zone ( 236 ) on the cathode side.

FIELD OF THE INVENTION

The present invention relates to fuel cells and is particularlyconcerned with gas separators between adjacent solid oxide fuel cells.

BACKGROUND OF THE INVENTION

The purpose of a gas separator in a fuel cell assembly is to keep theoxygen containing gas supplied to the cathode side of one fuel cellseparate from the fuel gas supplied to the anode side of an adjacentfuel cell, and to conduct heat generated in the fuel cells away from thefuel cells. The gas separator may also conduct electricity generated inthe fuel cells between or away from the fuel cells. Although it has beenproposed that this function may alternatively be performed by a separatemember between each fuel cell and the gas separator, much developmentwork has been carried out on electrically conductive gas separators.

Sophisticated ceramics for use in gas separators for solid oxide fuelcells have been developed which are electrically conductive, but thesesuffer from a relatively high fragility, low thermal conductivity andhigh cost. Special metallic alloys have also been developed, but it hasproved difficult to avoid the various materials of the fuel cellassembly and the interfaces between them degrading or changingsubstantially through the life of the fuel cell, particularly insofar astheir electrical conductivity is concerned, because of the tendency ofdifferent materials to chemically interact at the high temperatures thatare required for efficient operation of a solid oxide fuel cell. Forexample, most metallic gas separators contain substantial quantities ofthe element chromium, which is used to impart oxidation resistance tothe metal as well as other properties.

It has been found that where chromium is present in more than minutequantities it may combine with oxygen or oxygen plus moisture to formhighly volatile oxide or oxyhydroxide gases under conditions that aretypical of those experienced in operating solid oxide fuel cells. Thesevolatile gases are attracted to the cathode-electrolyte interface wherethey may react to form compounds that are deleterious to the efficiencyof the fuel cell. If these chromium reactions are not eliminated orsubstantially inhibited, the performance of the fuel cell deteriorateswith time to the point where the fuel cell is no longer effective.

Several of these metallic alloys and one proposal for alleviating thisproblem are described in our patent application WO96/28855, in which achromium-containing gas separator is provided with an oxide surfacelayer that reacts with the chromium to form a spinel layer between thesubstrate and the oxide surface layer and thereby tie in the chromium.However, these specialist alloys remain expensive for substantial use infuel cell assemblies and it would be preferable to have a lower costalternative.

Special stainless steels have also been developed that are stable athigh temperature in the atmospheres concerned, but they generallycontain substantial amounts of chromium to provide the desired oxidationresistance, and special coatings or treatments are required to preventthe chromium-based gases escaping from a gas separator formed of thesesteels. Another approach to a heat resistant steel gas separator isdescribed in our patent application WO 99/25890. However, all of theseheat resistant steels are specialist materials and their cost willremain high unless substantial amounts can be produced. Furthermore, thethermal and electrical conductivities of heat resistant steels are lowrelative to many other metals and alloys, for example 22-24 W/m.Kcompared to 40-50 W/m.K for the Siemens-Plansee alloy described inWO96/28855. To compensate for this, the thickness of the steel gasseparator has to be increased, increasing the mass and cost of a fuelcell stack.

In yet another proposal, disclosed in our patent application WO00/76015, we have found that copper-based gas separators may besuccessfully utilised in solid oxide fuel cell assemblies withoutpoisoning the anode. Such a gas separator member comprises a layer ofcopper or copper-based alloy having a layer of oxidation-resistantmaterial on the cathode side.

One of the major difficulties with developing a satisfactory gasseparator is ensuring that its coefficient of thermal expansion (“CTE”)is at least substantially matched to that of the other components of thefuel cell assembly. For example, solid oxide fuel cells comprising anoxide electrolyte with a cathode and an anode on opposed surfacesoperate at temperatures in excess of about 700° C., and the alternatinggas separators and fuel cells are generally bonded or otherwise sealedto each other. Thus, any substantial mismatch in the CTE between the twocomponents can lead to cracking of one or both of them, with resultantleakage of the fuel gas and oxygen-containing gas across the componentor components, and eventually to failure of the fuel cell stack.

A particular difficulty with developing a suitable fuel cell gasseparator is providing a material that provides all four functions ofseparating the fuel gas on one side from the oxygen-containing gas onthe other side, being thermally conductive, having a CTE substantiallymatched to that of the other fuel cell components, and beingelectrically conductive.

In order to meet these requirements, it has been proposed to provide agas separator formed principally of a material that may not beelectrically conductive, or not adequately electrically conductive, butthat meets the other requirements, and to provide electricallyconductive feedthroughs through the thickness of the separator. One suchproposal is made in Kendall et al. in Solid Oxide Fuel Cells IV, 1995,pp.229-235, in which the gas separator plate is formed of a zirconiamaterial and lanthanum chromite rivets extend through holes in theplate. Another proposal for electrically conductive feedthroughs throughthe thickness of the separator is made in EP 0993059. In this proposal,a ceramic gas separator plate, preferably stabilized zirconia, haspassages therethrough that in the preferred embodiment are filled withcathode material from the cathode side and with anode material from theanode side. Alternatively, they may be filled with a single materialcomposition such as doped chromite, silver-palladium or Plansee alloy.

Thus, the feedthrough material is different to that of the principalseparator material and will generally have a higher electricalconductivity. However, as the gas separator is subjected to thermalcycling, this can lead to the disadvantage of the feedthrough materialbecoming loose in the plate material, due to their different CTEs, andto the leakage of gas through the passages in which the feedthroughs areformed.

Additionally in EP 0993059, individual contacts for the feethroughs of,for example, Ni, Plansee metal or Ag—Pd on the anode side and Ag—Pd orlanthanum strontium manganite on the cathode side, are bonded to therespective electrode by means of a bond layer that overlies the entireelectrode surface. Such a bond layer will tend to inhibit free gas flowthrough the electrode and the individual contacts must be located veryaccurately on the electrodes to overlie the respective feedthrough whenthe fuel cell plates carrying the electrodes and the individual contactsare assembled with the gas separator plates

An alternative proposal published in US Patent Application 20020068677on 6 Jun. 2002 includes a gas separator plate in which the principalplate material is a high silica glass matrix having a metal conductorincorporated therein formed of a material such as silver, Ag—Pd alloy,gold and ferritic stainless steel.

An aim of each aspect the present invention is to provide a fuel cellgas separator that alleviates at least some of the abovementioneddisadvantages.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided afuel cell gas separator for use between two solid oxide fuel cells, thegas separator having a separator body with an anode-facing side and acathode-facing side and with paths of electrically conductive materialtherethrough from the anode-facing side to the cathode-facing side in anelectrode-contacting zone, wherein the electrically conductive materialforming at least part of the length of each path is a silver-glasscomposite.

By this aspect of the present invention, the advantage of separating thedesired level of electrical conductivity of the gas separator from thematerial of the separator body is achieved by the use of silver in theelectrically conductive paths through the separator body, and the riskof leakage of gases through the gas separator is alleviated by the useof glass in the electrically conductive paths. The glass may soften atthe operating temperature of the fuel cell and, if necessary, can flowwith expansion and contraction of the separator body as the separator issubjected to thermal cycling. The ductility of the silver facilitatesthis. The silver-glass composite may effectively be in the form of puresilver or a silver-based material in a glass matrix.

The material of the separator body is preferably selected with a CTEthat substantially matches that of other ftiel cell components, but anysuitable material may be selected, including electrically conductivematerials such as metals and alloys. In a solid oxide fuel cellassembly, in which the electrolyte material is preferably a zirconia andmay be the principal layer that supports the electrode layers, thematerial of the separator body is advantageously zirconia, as describedhereinafter.

The silver-glass composite preferably comprises from about 10 to about40 wt % glass, more preferably from 15 to 30 wt % glass. About 10 wt %glass is believed to be the lower limit to provide adequate sealingadvantages in the separator body, while at a level above about 40 wt %glass there may be insufficient silver in the composite to provide thedesired level of electrical conductivity. Potentially, the proportionsof silver and glass in the composite may be varied to best suit the CTEof the separator body but the major advantages of the composite lie inthe ability of the material to deform with expansion and contraction ofthe separator body and to conduct electricity.

The silver-glass composite may be formed by a variety of suitableprocesses, including mixing glass and silver powders, mixing glasspowder with silver salts, and mixing sol-gel glass precursors and silverpowder or silver salts. Alternatively, for example, the silver or silversalt may be introduced to the glass matrix after the glass particleshave been provided in the principal material of the gas separator, asdescribed hereinafter. The material is then fired. One suitable silversalt is silver nitrate. In a preferred embodiment the glass powder has aparticle size of less than 100 μm, most preferably with an averageparticle size in the range of 13 to 16 μm, and the silver powder has aparticle size less than 45 μm. A suitable binder is for example anorganic screen printing medium or ink. After mixing, the composition isintroduced to passages through the principal separator material andfired.

The silver may be commercially pure, a material mixture in which Ag isthe major component or, for example, a silver alloy.

Silver may advantageously be used alone in the glass matrix provided theoperating temperature of the fuel cell is not above about 900° C., forexample in the range 800 to 900° C. There may be some ion exchange ofthe silver at the interface with the glass that may strengthen theAg-glass bond and may spread interface stresses.

Particularly if the fuel cell operating temperature will be higher thanabout 900° C., above the melting point of the silver, for example up to1100° C., the silver may be alloyed with any suitable ductile metal ormetals having a sufficiently high melting point, for example one or morenoble metals such as gold, palladium and platinum. Preferably, therewill be no less than 50 wt % Ag present in the alloy. If the highmelting temperature alloying metal or metals excessively reduces theability of the silver alloy to bond with the glass by ion exchange atthe interface, a lower melting temperature metal such as copper may bealso included.

An alternative and cheaper material to combine with the Ag is stainlesssteel. The Ag and stainless steel may be mixed to powders prior to beingcombined with the glass.

A variety of different glass compositions can be used with the selectedprincipal separator material. The glass composition should be stableagainst crystallisation (for example, less than 40% by volumecrystallisation) at the temperatures and cool-down rates at which the,fuel cell gas separator will be used. Advantageously, the glasscomposition has a small viscosity change over the intended fuel celloperating range of, for example, 700 to 1100° C., preferably 800 to 900°C. At the maximum intended operating temperature, the viscosity of theglass should not have decreased to the extent that the glass is capableof flowing out of the separator body under its own weight.

Preferably, the glass is low (for example, less than 10 wt %) in or freeof fuming components, for example no lead oxide, no cadmium oxide, nozinc oxide, and no or low sodium oxide and boron oxide. The type ofglasses that exhibit a small viscosity change over at least the 100° C.temperature range at the preferred fuel cell operating range of 800° C.to 900° C. are typically high silica glasses, for example in the range55 to 80 wt % SiO₂. Such glasses generally have a relatively low CTE.

Preferred and more preferred compositions of such a high silica glass,particularly for use with a zirconia gas separator body, are set out inTable 1. TABLE 1 Preferred Range More Preferred Range Oxide wt % wt %Na₂O   0-5.5   0-2.0 K₂O  8-14   8-13.5 MgO   0-2.2   0-0.05 CaO 1-3  1-1.6 SrO 0-6 0.5-1   BaO 0-8   0-4.4 B₂O₃  6-20  6-20 Al₂O₃ 3-7  3-6.0 SiO₂ 58-76 60-75 ZrO₂  0-10   0-5.0

The electrically conductive material may be introduced to the paths byany suitable means. For example, after the glass powder or particleshave been introduced to the paths or perforations, a solution of asilver salt or very fine suspension of the silver material, for exampleas a liquid coating applied to one or both surfaces of the separatorbody, may be permitted or caused to be drawn through the glass particlesin the paths or perforations, such as by capillary action.Alternatively, the solution or suspension could be injected in. Morepreferably, a mixture of the glass and silver material powders in abinder is printed, for example by screen or stencil printing, onto oneor both surfaces of the separator body to at least partly fill the pathsin the body. The mixture is then heated to melt the glass and sinter thesilver. The molten glass-silver composite then flows in the paths toseal them. A suitable heating/firing temperature is dependent upon theglass composition and the silver material but is preferably in the range650 to 950° C. for pure silver in a high silica glass matrix for optimummelting of the glass without undue evaporation of the silver.

In order to ensure that the fuel cell gas separator does transmitelectricity between the surfaces defined by the anode-facing andcathode-facing sides of the separator body, the silver-glass compositein the paths may extend to the outer surfaces of the separator body.Alternatively, the silver-glass composite may have an electricallyconductive coating on it in the paths which extends to the respectivesurface and which may protect the silver-glass composite and/or theinterface between the gas separator and the adjacent electrode. Forexample, in accordance with the second aspect of the invention, a Niprotective coating may be provided at the anode side, optionally with anundercoating of Ag, and a Ag or Ag alloy such as Ag—Sn protectivecoating may be provided at the cathode side to alleviate loss of thesilver-glass composite through evaporation or “wiclcing” to other nearbycomponents. In particular, the coating may alleviate loss of the glassin the silver-glass composite to the adjacent fuel cell electrode orother porous component by capillary action at the fuel cell operatingtemperature. The coatings also enhance electrical contacts and provide adegree of compliance.

To enhance electrical current flow between the adjacent fuel cell andgas separator, the aforementioned protective coatings advantageouslyextend across the electrode-contacting zones of the separator body, forexample with a thickness in the range of 10 to 1000 μm, preferably 60 to150 μm. Alternatively or in addition, a respective mesh or other currentcollector may be interposed between the gas separator and the electrodesof the adjacent fuel cells. The mesh or other current collector maydefine, or partly define, gas passages through which the air or otheroxygen-containing gas on the cathode side of the gas separator and thefuel gas on the anode side of the gas separator is passed over theadjacent fuel cell electrode.

According to a second aspect of the invention, there is provided a fuelcell gas separator for use between two solid oxide fuel cells, the gasseparator having a zirconia-based body with an anode-facing side and acathode-facing side and with paths of electrically conductive materialtherethrough from the anode-facing side to the cathode-facing side in anelectrode-contacting zone of the separator body, wherein theelectrically conductive material forming at least part of the length ofeach path is silver or a silver-based material and wherein a coating ofnickel on the electrode-contacting zone on the anode-facing sideoverlies said silver or silver-based material in the paths ofelectrically conductive material and a coating of Ag or of Ag—Sn alloyon the electrode contacting zone on the cathode-facing side overliessaid silver or silver-based material in the paths of electricallyconductive material.

By this aspect of the invention, the electrically conductive paths areprotected by the opposed surface coatings over the electrode contactingzones of the separator body, electrical contact with an integral ofseparate device or devices between the gas separator and the adjacentelectrode for current collection and/or gas flow control may beenhanced, and the coatings may give a degree of compliance bydistributing uneven loads due to components of the fuel cell stackhaving slightly different heights.

By the term “electrode-contacting zone” as used throughout thisspecification is meant the portion of the gas separator body that isopposed to and aligned with the respective electrodes of the adjacentfuel cell plates. Any contact of the electrode-contacting zone with theadjacent electrodes may be indirect, through interposed currentcollection and/or gas flow control devices. It will be understoodtherefore that the use of the term “electrode-contacting zone” does notrequire that zone of the gas separator body to directly contact theadjacent electrodes.

The silver or silver-based electrically conductive material may be thesilver-glass composite used and described with reference to the firstaspect of the invention, but could alternatively be metallic silver(commercially pure), a metallic mixture in which Ag is the majorcomponent, or a silver alloy.

Particularly if the fuel cell operating temperature will be higher thanabout 900° C., above the melting point of the silver, for example up to1100° C., the silver may be alloyed with any suitable ductile metal ormetals having a sufficiently high melting point. Examples of such metalsare one or more noble metals such as gold, palladium and platinum.Preferably, there will be no less than 50 wt % Ag present in the alloy.An alternative and cheaper material to combine with the Ag is stainlesssteel. The Ag and stainless steel may be mixed as powders and sinteredtogether by firing in the paths through the separator body.

The metallic silver, silver mixture or silver alloy electricallyconductive material may be introduced to the pores by any suitablemethod, including screen or stencil printing a slurry of the metal,mixture or alloy in an organic binder into the paths, or coating thesurfaces of the electrode-contacting zone by, for example, printing,vapour deposition or plating and causing the coated metal, mixture oralloy to enter the paths.

The layer of nickel, preferably commercially pure nickel, on theanode-facing side may have a thickness in the range of about 10 to 1000μm, preferably 60 to 100 μm. To ensure continued contact of the Ni layerwith the separator body during extended thermal cycling of the fuel cellstack particularly where the separator body is zirconia, a layer ofsilver, preferably commercially pure Ag, may be disposed on theelectrode-contacting zone between the coating of nickel and theanode-facing side of the gas separator body. Such a layer of silver mayhave a thickness in the range of about 10 to 1000 μm, preferably 20 to200 μm, and conveniently provides enhanced compliance of the overallcoating on the anode side due to its ductility.

Ag—Sn alloy on the catlhode-facing side of the separator body preferablycontains from about 4 to about 20 wt % Sn, and may have a thickness inthe range of about 10 to 1000 μm, preferably 100 to 150 μm.

An Ag—Sn alloy coating may have a surface layer of SnO₂, formed forexample in the oxidising atmosphere on the cathode-facing side of thegas separator. The SnO₂ surface layer alleviates loss of Ag by“evaporation” at the elevated temperatures of use of the gas separator.To improve the electrical conductivity of the coating on thecathode-facing side, the coating must include up to about 10 wt % ofdopants such as Pd and La. Each of the coating layers may be applied byany suitable means, including screen printing, spin coating, a vapourdeposition process such as magnetron sputtering, slurry coating and tapecasting.

As an alternative to the Ag—Sn coating, a layer of silver may be formedon the cathode-facing side of the separator body. The silver coating onthe cathode side may have a thickness of 10 to 1000 μm, preferably 50 to250 μm.

The following description applies to both aspects of the invention, asdoes the above discussion of surface coatings if they are provided onthe gas separator of the first aspect of the invention.

The zirconia of the gas separator may be yttria-stabilised, for example3 to 10 wt % Y. Alternatively, the zirconia may include other materialswhile retaining a zirconia-based structure. For example, the zirconiamay be a zirconia alumina having up to 15 wt %, or even up to about 20wt %, alumina. For convenience, all such materials are hereinafterreferred to as zirconia.

The thickness of the separator body is preferably no more than 500 μm,more preferably substantially less than this in order to minimize theoverall thickness or height and mass of a full cell stack utilizing thegas separator or separators, for example in the range 50 to 250 μm.While a lesser thickness could be used, the gas separator becomesdifficult to manufacture and it becomes more difficult to ensure thatthe material of the separator body is dense, that is that it is gastight to the gases in the fuel cell assembly. Greater thicknesses may beused but are unnecessary, and more preferably the thickness is no morethan 200 μm.

The separator body may be formed by any suitable means, dependingparticularly upon the material and the shape of the separator. A gasseparator for use with a planar fuel cell will generally be in the formof a plate, and a zirconia plate, for example, may be formed by tapecasting the green material and sintering. Suitable manufacturing methodsmay be readily identified and do not form part of the present invention.The separator body may be formed in two or more layers, for example ofzirconia, that may be separated by a layer of electrically conductivematerial in contact with the paths of electrically conductive materialthrough the layers of the separator body. Preferably the electricallyconductive material in the paths and the separating layer is the same.

As noted already, the gas separator must be gas tight to the gases usedin the fuel cell assembly, and most preferably the material of theseparator body is dense. However, the material could be porous, with theelectrically conductive material plugging the pores through thethickness of the material. Preferably, however, the paths ofelectrically conductive material are defined by perforations through theseparator body.

For convenience, such perforations preferably extend substantiallyperpendicularly through the thickness of the separator body. However,this is not essential and it may be advantageous for the paths ofelectrically conductive material to be inclined to the perpendicular.Each path at the anode-side of the separator body may be offset relativeto a connected path at the cathode-side to further alleviate the risk ofleakage of gases through the separator, and/or the separator body may beformed as two or more layers separated by a layer or layers ofelectrically conductive material that may be the same as or different tothe electrically conductive material in the paths through the separatorbody, as described above.

Each path of electrically conductive material through the separator bodypreferably has a diameter or average cross-sectional dimension in therange of 50 to 1000 μm. If the paths are defined by perforations, theperforations may be formed during manufacture of the separator body orsubsequently, for example by laser cutting. The minimum size of theperforations is a function of the difficulty of forming them andplugging them with the electrically conductive material. Morepreferably, the average cross-sectional dimension is in the range 200 to400 μm, for example about 300 μm.

The minimum number of perforations is a function of their size, theelectrical conductivity of the plug material and the electrical currentto be passed through the gas separator. If the perforations have anaverage cross-sectional dimension towards the upper end of the preferredrange, they may be fewer in number and more widely spaced. Preferably,the total area of the paths of electrically conductive material throughthe separator body is in the range of 0.1 mm² to 20 mm² per 1000 mm²surface area (measured on one side only) of the electrode-contactingzone of the separator body, more preferably in the range 0.2 mm² to 5mm² per 1000 mm². In a currently preferred embodiment, there are 19paths of electrically conductive material having an average diameter ofabout 300 μm through a gas separator plate having anelectrode-contacting zone or functional gas separating area of about5400 mm².

Advantageously, the paths of electrically conductive material alsoprovide thermally conductive paths for transmission of heat away fromthe fuel cells on opposite sides of the gas separator.

Surface formations may be provided in the electrode-contacting zone ofthe gas separator to define gas flow passages, optionally in conjunctionwith a separate current collector. The surface formations may be in theform of parallel ridges which may be integrally formed with theseparator body, or may be affixed to the surfaces of the separator body.The surface formations may have any suitable height to provide for thenecessary gas flow, for example up to about 750 μm, preferably about 500μm high.

Advantageously, in one embodiment, the electrically conductive paths inthe separator body are covered by an array of parallel ridges on bothsides, which extend parallel to the desired direction of the gas flow.The ridges on opposed sides of the gas separator may extend parallel toeach other or perpendicularly to each other, depending upon whether thefuel gas and oxygen-containing gas are to be in co- or counter-flow, orin transverse- or cross-flow. The ridges may be formed of any suitablematerial that is electrically conductive and structurally and chemicallystable in the fuel cell operating environment, and at least in thesecond aspect of the invention are conveniently bonded to the Ni and thesilver or Ag—Sn coatings on the separator body, or possibly through theNi coating to the Ag undercoating if it is present. In one embodiment,the ridges on each side of the gas separator are made of the samematerial as the respective electrode that they contact. Thus, on thecathode side the ridges may be formed of a conductive perovskite such aslanthanum strontium manganate, preferably coated with a metallic silvercoating up to about 100 μm, preferably about 50 μm, thick. On the anodeside, the ridges may be formed of a nickel-zirconia cermet, preferablywith a metallic nickel coating up to about 100 μm, preferably about 50μm, thick.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of a fuel cell gas separator plate in accordancewith the invention will now be described by way of example only withreference to the accompanying drawings in which:

FIG. 1 is a schematic exploded side view of a fuel cell stackincorporating one embodiment of a gas separator plate in accordance withthe invention;

FIG. 2 is a schematic side view of one of the fuel cell gas separatorplates of FIG. 1 during its manufacturing process;

FIG. 3 is a plan view of a fuel cell stack incorporating anotherembodiment of gas separator plates in accordance with the invention;

FIG. 4 is an exploded schematic perspective view looking downwards andillustrating the general orientation of cell plates and gas separatorplates within the stack shown in FIG. 3;

FIG. 5 is a schematic perspective view looking upwards at the cellplates and gas separator plates in the same exploded positions shown inFIG. 4;

FIG. 6 is a perspective view of the top side of one of the cell platesshown in FIG. 4;

FIG. 7 is a cut-away perspective view of the top side of one of the gasseparator plates shown in FIG. 4;

FIG. 8 is a cut-away underside view of the gas separator plate shown inFIG. 7;

FIG. 9 is a diagrammatic cross-sectional view through a portion of a gasseal assembly between the plates shown in FIGS. 3 and 4; and

FIG. 10 is an exploded perspective view of another embodiment of gasseparator plate in accordance with the invention, shown with one of twoassociated fuel cell plates.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring to FIG. 1 there is shown an array 10 of alternating fuel cells12 and gas separator plates 14 in accordance with the invention. Thefuel cells 12 are planar and comprise a solid oxide electrolyte supportlayer 16 with an anode layer 18 on one side and a cathode layer 20 onthe other side. The electrolyte is preferably yttria-stabilized zirconiasuch as 3Y, 8Y or 10Y. The anode is preferably a nickel-zirconia cermetand the cathode is preferably a conductive perovskite such as lanthanumstrontium manganate. Such solid oxide fuel cells are well known and willnot be described further.

Each gas separator plate 14 has a three-layer sandwich structure withfirst and second outer layers 22 and 24, and a third intermediate layer26.

The first and second layers are conveniently formed of a zirconia tosubstantially match the CTE of the electrolyte support layer 16 of thefuel cells 12. The zirconia may be yttria-stabilized, but could be, forexample, an alumina-added zirconia with up to 20 wt % alumina.

The zirconia is not electronically conductive, and each of the first andsecond layers 22 and 24 has a plurality of perforations 28 extendingperpendicularly through its thickness which are filled with silver orsilver-based electrically conductive plug material. In the preferredembodiment, the plug material is a composite of 80 wt % silver in glass.The silver is commercially pure and the glass has a composition of0.8-1.2 wt % Na₂O, 8.9-9.2 wt % K₂O, <0.1 wt % MgO, 1.4-1.5 wt % CaO,<0.1 wt % SrO, 3.1-4.2 wt % BaO, 7.2-10.2 wt % B₂O₃, 6.2-6.6 wt % Al₂O₃,and 68.8-70.4 wt % SiO₂.

The perforations 28 in the first and second layers in each gas separatorplate 14 are offset so that no perforation in the first layer iscoincident with a perforation in the second layer. The material of thethird intermediate layer 26 is the same as the plug material in thefirst and second layers, and has a thickness of less than 60 μm.

The perforations have an average cross-sectional dimension of about 300μm, and the plug material 30 seals the perforations to present a totalcross-sectional area of plug material in the range of 0.2 to 5 mm² per1000 mm² of surface area of the electrode-contacting zone measured onone side only of each of the first and second layers.

FIG. 2 illustrates schematically one method of forming the gas separatorplate 14. In this method, a thicker layer of a precursor of the materialof the third intermediate layer, for example about 200 μm, is screenprinted onto the inner surface of the second layer 24. The screenprinting may be performed at or near room temperature. The precursor isa mixture formed by mechanical agitation of powdered glass having aparticle size of less than 100 μm and an average size range of 13 to 16μm and silver metal powder having a particle size range of less than 45μm in binder. A suitable binder system is a combination of a screenprinting inks available under the brand name CERDEC and DURAMAX.

As shown in FIG. 2, some of the coated material enters the perforations28 in the second layer during the coating process. The first layer 22 isthen superposed onto the coated precursor material of the third layer onthe second layer, with the perforations 28 in the first layer offsetrelative to the perforations 28 in the second layer. Sufficient pressureis then applied through the first layer 22, as represented by thedownwards arrows in FIG. 2, to cause the precursor material to flow.This reduces the precursor material of the third layer to the desiredthickness and forces the composite material further into theperforations 28 of the second layer 24 as well as into the perforations28 in the first layer 22 to act as the electrically conductive plugs.The gas separator plate is then fired at a temperature of about 850 to920° C. to melt the glass and sinter the silver into a continuouselectrically conductive path in the glass matrix. At the operatingtemperature of the fuel cell, the glass in the composite is a viscousfluid and forms a gas barrier, while the silver provides the electricalconductivity. At low temperatures, or at shutdown, the molten/viscousglass in the composite returns to a solid/rigid state. Should thecomposite material become damaged in this condition, once it returns tooperating temperature the glass returns to a viscous fluid state andwill recover its sealing properties.

A similar forming process may be adopted when the electricallyconductive material comprises silver metal, a silver mixture or silveralloy, with the metal or alloy powder or powders being formed into aslurry and screen printed.

The outer surfaces of the gas separator plate 14 are then coated withconductive layers of an Ag—Sn alloy on the cathode side and nickel metalon the anode side, optionally with a layer of silver between the anodeside and the nickel coating, to protect the electrically conductivepaths. A layer of SnO₂ may overlie the Ag—Sn coating. The ductility ofthe coatings may alleviate stresses arising from uneven loaddistribution in the fuel cell stack due to slightly different heights ofthe components. The coatings may also act to fill the perforations 28from the outside and alleviate wicling of the glass into adjacent porouslayers of the fuel cell stack, so as to ensure that electricallyconductive paths are provided via the perforations and the thirdintermediate layer from one of the outer surfaces to the other outersurface of the gas separator plate 14, as represented by the upwardsarrows in FIG. 1. Additional detail on the application processes for thecoatings is given below.

Referring to FIG. 1 again, one current collector 32 is illustratedschematically between the upper pair of fuel cell 12 and gas separatorplate 14, and this may define gas flow passages between the twostructures. Such gas flow passages are necessary between each pair ofadjacent gas separator plates and fuel cells, but are omitted from FIGS.1 and 2 for convenience only. They are conveniently in the form ofridges on the outer surfaces of the gas separator plates 14, over any ofthe aforementioned coatings, as illustrated in and described withreference to FIGS. 3 to 9.

Referring to FIGS. 3, 4 and 5 a solid oxide fuel cell stack assembly 102comprises a stack 103 of alternating fuel cell plates 110 and gasseparator plates 130 held within a tubular housing 104. All of the cellplates 110 are identical and all of the separator plates 130 areidentical. As with the plates in FIGS. 1 and 2, typically there might be20 to 500 of each of these plates in the stack. Fuel gas and air aresupplied at one axial end of the stack assembly and exhaust gases arecollected at the other end in a co-current manifolding system. Eitherend is suitable for the supply and the exhaust functions, but themanifold system may alternatively be counter-current. In the describedco-current embodiment, the fuel and air supplies are both at the bottomand the exhausts are at the top, but in many circumstances it ispreferred for the fuel to be supplied from the bottom and the air to besupplied from the top in a counter-current arrangement. Alternatively,all of the gas supplies and exhausts may be at the same end.

Referring to FIGS. 3 to 8, each cell plate 110 has a substantiallycentral, square anode layer on an upper face of the electrolyte-basedcell plate and a substantially central, square cathode layer on a lowerface of the cell plate to form a substantially square fuel cell 112.

The cell plates 110 and separator plates 130 have the same outer shape,which could be described as generally trilobular, or part way between acircle and a triangle. The shape could alternatively be described asgenerally circular with three rounded lobes extending therefrom. Two ofthe lobes 174 and 176 are the same size and the third lobe 172 extendsabout 50% further than the others circumferentially around the peripheryof the plate. At each lobe 172, 174 and 176 a kidney shaped aperture(numbered 114, 116 and 118 in the cell plate and 115, 117 and 119 in theseparator plate respectively) extends through the plate. The largerlobes 172 carry the larger apertures 114 and 115. A system of rib orridge-shaped seals on the faces of the plates directs the gas flowswithin the stack. These seals are described hereinafter in more detail,but it will be appreciated that other types of seals may be utilised,including gasket seals.

Fuel distribution and exhaust collection manifolds 105 and 106,respectively, (see FIG. 3) defined by the three aligned series ofapertures 114 and 115, 116 and 117, and 118 and 119 in the fuel cell andgas separator plates and formed by interlocking the seal components ofthe plates 110 and 130, conduct the fuel inlet and exhaust streams pastthe air side of the plates to the anode side. Air supply and collectionmanifolds 107 and 108 respectively are created by three volumes formedbetween the periphery of the stack 103 and the inside wall of thehousing 104. Manifold 107 is formed essentially between the lobes 174and 176 of the plates, and the two exhaust manifolds 108 are formedessentially between the lobes 172 and 174 and the lobes 172 and 176,respectively, of the plates. Air inlet manifold 107 has an angularextent that is about 50% larger than the each of the two exhaustmanifolds 108, and is opposite the fuel inlet or distribution manifold105. Respective elongate, sliding fibrous seals 109 extend along thestack adjacent the lobes 172, 174 and 176, between the stack 103 and theinside wall of the housing 104 to separate the air supply manifold 107from the two air collection manifolds 108. The fibrous seals may permita degree of leakage between the manifolds 107 and 108, but this is notlikely to be detrimental to the operation of the stack.

The housing 104 is constructed of a suitable heat resistant steel sheetmaterial, which may be lined with a suitable insulating material, and isslid into position over the stack 103 after the plates 110 and 130 havebeen assembled together.

In operation of the stack, fuel gas flows up through the larger aperture114 defining an inlet port in each cell plate 110 and (at arrow A)across the face of the fuel cell anode, then divides its flow (arrows Band C) to exit up through exhaust port apertures 117 and 119,respectively, in the adjacent gas separator plate 130. On the oppositeface of the cell plate 110 air, which has passed up the side of thestack 103 through the inlet manifold 107 between the stack and thehousing, flows in (arrow D) from the periphery of the stack 103 andacross the face of the fuel cell cathode, in counter-current to the fuelgas flow across the fuel cell anode, before dividing its flow to exit(arrows E and F) from the periphery of the stack 103 and then continuingup through the exhaust manifolds 108 to the top of the stack.

Referring to FIG. 6, the generally planar cell plate 110 used in thestack 103 is shown in greater detail. The square fuel cell 112 on theplate (the anode is visible) has an electrolyte supported structure withthe electrolyte material extending out to form the main body of theplate 110. The electrolyte is preferably a yttria stabilised zirconiaand suitable 3Y, 8Y and 10Y materials are known to those in the art. Theanode is preferably a nickel-zirconia cermet and the cathode ispreferably a conductive perovskite such as lanthanum strontiummanganate. The underside of the cell plate 110 and the cathode arevisible in FIG. 5.

A pair of parallel ribs 120 and 121 project from the planar surface 124of the cell plate 110 forming a valley or groove 122 therebetween. Thesurface 124 is the upper, anode surface of the cell plate when the stackis oriented for use. The ribs are formed of zirconia and may beintegrally formed with the main body of the plate or may be formedseparately, for example from a screen printed slurry, and be fired intointegral relationship with the main body. Each rib 120 and 121 forms acontinuous path or closed loop outwardly of the apertures 114, 116 and118 through the cell plate and around the perimeter of the region whichthe fuel gas is permitted to contact. In particular, the closed loopdefined by the ribs 120 and 121 is waisted alongside the anode to directfuel gas from the inlet aperture 114 over the anode.

In all of FIGS. 3 to 9, the thickness of the plates 110 and 130 and theheight of the ribs are shown greatly exaggerated to assist theexplanation of the components. In this embodiment, the fuel cell 112 is2500 mm², the cell plate is 150 μm thick and the ribs are approximately500 μm high, 1 mm wide and approximately 2 mm apart.

On the lower, cathode side 154 of each fuel cell plate 110, as shown inFIG. 5, a respective single rib 134 (that corresponds to the ribs 120and 121 in term of size and how it is formed) extends from the planarsurface 154 around each of the apertures 114, 116 and 118 through theplate. Each of the ribs 134 around the apertures 116 and 118 has an arm135 that extends inwardly and towards the aperture 114 (but shortthereof) alongside the cathode layer of the fuel cell 112 to assistguidance of incoming air over the cathode. One of the ribs 134 is alsoshown in FIG. 9 and the use of the rib seals is described with referenceto that Figure.

FIGS. 7 and 8 show the planar gas separator plate 130 in greater detail.In FIG. 7, the surface 133 is the upper, cathode-contacting surface ofthe separator plate 130 when the stack is oriented for use. Respectivepairs of parallel ribs 136 and 137 project from the planar surface 133of the separator plate 130 forming valleys or grooves 138 therebetween.The pairs of parallel ribs 136 and 137 correspond to the ribs 120 and121 in terms of size, spacing and how they are formed, but extend aroundthe apertures 115, 117 and 119 through the plate 130 to cooperate withthe ribs 134 on the cathode-side of the adjacent fuel cell plate 110.The respective ribs 136 associated with the apertures 117 and 119 eachhave a double-walled arm 139 that is closed at its distal end tocooperate with and receive the arm 135 of the corresponding rib 134.

On the lower, anode-contacting side 132 of each gas separator plate 130,a single rib 142 is shown in FIG. 8 and partly in FIG. 7. The rib 142corresponds to the ribs 120 and 121 in terms of size and how it isformed, and forms a continuous path outwardly of the apertures 115, 117and 119 through the plate 130 and around the perimeter of the regionthat the fuel gas is permitted to contact. The outline of the rib 142corresponds to the groove 122 between the ribs 120 and 121 on the anodesurface of the adjacent fuel cell plate 110 and cooperates with thoseribs in forming a seal.

As explained hereinafter, with reference to FIG. 9, glass sealant 140 isused in both of valleys 122 and 138 to form a seal between the ribs.

Each separator plate 130 is manufactured from a zirconia tosubstantially match the CTE of the main body of the cell plates 110.This greatly minimises thermal stresses in the assembly during start-up,operation and shut-down. The zirconia may be yttria-stabilised, butcould be, for example, an alumina-added zirconia with up to 20 wt %alumina, preferably up to 15 wt % alumina.

The zirconia is not electrically conductive, and the separator plate 130has an array of perforations 150 extending perpendicularly through itsfull thickness that are filled with an electrically conductive plugmaterial. These perforations may be formed by laser cutting and occupy aregion in the plate 130 which is directly opposite the region occupiedby the fuel cell 112 in plate 110. The plug material may be metallicsilver (commercially pure) which is plated into the perforations bystandard plating or printing techniques. Alternatively the perforationsmay be filled with a silver mixture, a silver alloy or a silvercomposite, such as a composite of silver, silver mixture or silver alloyin glass. Suitable alloying elements or materials include gold,palladium and platinum. Alternatively, the silver may be mixed withstainless steel, for example as powders prior to sintering in theperforations. In the preferred embodiment, the perforations are filledwith a silver-glass composite of the type and in the manner describedwith reference to FIGS. 1 and 2 (except for the compression step, as theseparator plate 130 is formed of a single layer of zirconiacorresponding generally to the layer 24 in FIGS. 1 and 2).

The perforations have an average cross-sectional dimension of about 300μm, and the plug material seals the perforations to present a totalcross-sectional area of plug material in the range of 0.2 to 5 mm² per1000 mm² of the electrode-contacting zone (measured on one side only ofthe plate 130). The electrically conductive silver based plug whichfills each perforation is plated with a protective Ni coating on theanode side and an Ag or Ag—Sn coating on the cathode side. The coatingsextend over the entire electrode contacting zone of the plate. Thenickel coating may have an undercoating of Ag to assist the Ni to bond.to the separator body and to enhance the compliance of the anode-sidecoating. An Ag—Sn coating may have an SnO₂ surface layer as a result ofbeing oxidised. Such coatings, for example by screen printing of powdermaterials in a binder and subsequent firing, may act to fill theperforations 150 from the outside so as to ensure that electricallyconductive paths are provided via the perforations from one of the outersurfaces to the other outer surface of the gas separator plate. By wayof example only, the nickel coating, the Ag undercoating (if present)and the Ag—Sn coating may have thicknesses in the range of 60-100 μm,20-200 μm and 100-150 μm, respectively. The alternative Ag coating onthe cathode side may have a thickness in the range of 50 to 250 μm.

The coating materials may be applied by any suitable process to achievethe required thickness and consistency, including screen printing apaste of the metal, metals or alloy powder made using a suitable binder,spin coating using a suspension of the metal, metals or alloy powder, aphysical vapour deposition process such as magnetron sputtering, slurrycoating or tape casting.

In a particular embodiment, a nickel coating having a thickness of about80 μm was formed on the anode side of the electrode-contacting zone byscreen printing a paste of Ni powder in a suitable commerciallyavailable organic binder with no Ag undercoating, and then firing the Nilayer. Initially, the Ni layer was oxidised on a first firing. During asubsequent firing in a reducing atmosphere such as hydrogen or fuel gas,the Ni oxide was reduced and the Ni layer actively bonded to theelectrode-contacting zone on the anode side of the gas separator plateto lower the contact resistance, protect the plug material in the seals,and provide a degree of compliance within the stack as a result of theductility of the nickel layer.

On the cathode side in the particular embodiment, an Ag—Sn alloy layerhaving a thickness of about 140 μm was produced on theelectrode-contacting zone by screen printing a paste of the alloy powderin an organic binder. The alloy powder contained 8 to 10 wt % Sn in Ag.The screen printing was followed by heating the coating to a temperaturein the range of 500 to 950° C. in an oxidising atmosphere, during whicha continuous SnO₂ surface layer was formed on the coating. As with thenickel layer, the Ag—Sn alloy layer protects the plug material in theperforations, lowers the contact resistance of the gas separator plateand provides a degree of compliance due to its ductility.

An array of parallel ridges 148 is positioned parallel to the air flowstream in the electrode contacting zone on the cathode side 133 of eachplate 130. These ridges 148 are each aligned over a corresponding row ofperforations 150 and over the Ag—Sn coating. To assist explanation,about half of the ridges 148 have been removed in FIGS. 4 and 7. Theridges 148 perform two major functions. First they provide a conductivepath between the plug material in perforations 150 and Ag—Sn coating andthe fuel cell 112. Second they provide physical support to brace thethin and fragile cell plate as well as means for distributing gas flowsin the narrow spaces between the cell plates and the separator plates.The ridges 148 thus need to be both electrically conductive andstructurally stable. The ridges 148 are approximately 500 μm high andcould be made from a conductive perovskite, such as the LSM material ofthe cathode, optionally with a metallic silver coating about 50 μm thickover the ridges.

On the underside of plate 130 (ie. surface 132 shown in FIG. 8), therows of plugged perforations 150 and the Ni coating are covered by anarray of parallel ridges 162 that are positioned parallel to the fuelgas flow stream. Again, about half the ridges 162 are cut away in FIGS.5 and 8 to assist visualisation of the structure. The ridges 162 performas a current collector whereby current is conducted between the plugmaterial in the perforations 150 and the Ni coating and the anode. Theyalso provide physical support for the cell plate and additionallyprovide means for directing and distributing gas flows in the narrowspaces between the cell plates and separator plates. The ridges areapproximately 500 μm high and could be formed from the same material asthe anode, optionally with an overlay (approx 50 μm thick) of nickel.

Referring to FIG. 9, a pool of glass sealant 140 is located in thevalley 138 between the ribs 136 and 137 and is pressed into by rib 134.Each rib has a tapered profile with oppositely inclined flanks and adistal surface. A similar arrangement applies between the ribs 120 and121 and rib 142, but will not be described separately. Duringmanufacture, the glass is introduced as a powder and the stack assembledbefore the stack is heated to melt the glass in order to form therequired seal. Thus, no binder is required. In operation of the stackthe glass sealant 140 is fully molten but highly viscous and is retainedin the valley 138 by one of the following three options not shown inFIG. 9. The glass advantageously has a composition range of 0-0.7 wt %Li₂O, 0-1.2 wt % Na₂O, 5-15 wt % K₂O, 0-2 wt % MgO, 2-8 wt % CaO, 0-2 wt% SrO, 2-12 wt % BaO, 2-10 wt % B₂O₃, 2-7 wt % Al₂O₃, 50-70 wt % SiO₂and 0-2 wt % ZrO₂.

In one embodiment option, the distal surface peak 151 of the rib 134contacts the floor 156 of the groove 138 leaving at least one of theflanks 152 of the rib 134 clear of the flanks 158 of the groove andleaving the distal surfaces 160 and 161 of ribs 136 and 137 clear of thebasal surface 154 of the plate 110. In this case the glass sealant 140would be retained by surface tension between the spaced flanks 152 and158.

In a second, and preferred, embodiment option, the distal surfaces 160and 161 contact the basal surface 154 leaving at least one of the flanks152 clear of the flanks 158 and the distal surface 151 clear of thefloor 156. In this case the sealant 140 would be retained between thedistal surface 151 of the rib 134 and the floor 156, with some displacedoutwardly to between the spaced flanks 152 and 158.

In a third embodiment option, both flanks 152 would engage correspondingflanks 158 leaving the distal surfaces 160 and 161 clear of the basalsurface 154 and the distal surface 151 clear of the floor 156. In thiscase the sealant 140 would fill the volume between the distal surface151 and the floor 156.

Referring now to FIG. 10, there is shown (in exploded manner) a fuelcell plate 210 superposed over a gas separator plate 212. In use, theplates 210 and 212 are in at least substantially face to face contactand there would be a stack of alternating fuel cell plates 210 and gasseparator plates 212.

The plates 210 and 212 are seen in perspective view from above with acathode layer 214 visible on an electrolyte layer 216 on the fuel cellplate 210. An anode layer (not visible) corresponding to the cathodelayer 214 is provided on the underside (in the drawing) of the fuel cellplate.

The fuel cell and gas separator plates 210 and 212 are generallycircular and are internally manifolded with a fuel inlet opening 218, afuel outlet opening 220, air inlet openings 222 and air outlet openings224, which respectively align when the plates are stacked. A gasket-typeseal 226 and 228, respectively, is provided on the upper face (in thedrawing) of each of the fuel cell and gas separator plates 210 and 212.The gasket-type seals 226 and 228 are conveniently formed of a glasscomposition or a glass composite.

The seal 226 has air inlet ports 230 associated with the air inletpassages and air outlet ports 232 associated with the air outletpassages 224 to permit air to flow across the cathode layer 214 betweenthe cathode and the adjacent gas separator plates (not shown). The seal226 extends wholly around the fuel inlet passage 218 and outlet passage220 to prevent fuel flowing over the cathode side of the fuel cell plate210.

Correspondingly, the seal 228 on the gas separator plate 212 extendswholly around the air inlet passages 222 and the air outlet passages224, but only around the exterior of the fuel inlet passage 218 andoutlet passage 220 so that fuel gas can flow from the fuel inlet passage218, across the anode, between the fuel cell plate 210 and adjacent gasseparator plate 212, before exiting through the fuel outlet passage 220.

Means (not shown) is provided to distribute the reactant gas across therespective electrode and to provide support for all of the plates 210and 212 in a fuel cell stack. Such means may be in the form of surfaceformations on the gas separator plate 212, for example as described withreference to FIGS. 3 to 9, or on the fuel cell plate 210. Alternatively,the gas may be distributed by a separate member between the plates, suchas a mesh or corrugated structure, that may also act as a currentcollector.

As before, the cathode material is preferably a conductive perovskitesuch as lanthanum strontium manganate that is porous, and the anode ispreferably formed of a porous nickel-zirconia cermet.

The electrolyte layer 216 is preferably a yttria-stabilized zirconiasuch as 3Y, 8Y or 10Y and extends beyond the electrode layers to definethe internally manifolded fuel and air inlet and outlet passagestherethrough, to support the seal 226 and to provide a contact surfacefor the seal 228 on the gas separator plate 212.

The gas separator plate has a similar profile to the fuel cell plate 210and is advantageously also formed of a zirconia to substantially matchthe CTE of the electrolyte layer 216 of the fuel cell. The zirconia ofthe gas separator plate 212 may be yttria-stabilized, but could be, forexample, an alumina-added zirconia with up to 20 wt. % alumina.

Since the zirconia is non-electrically conductive and one of thefunctions of the gas separator plate 212 is to transmit electricalcurrent from one fuel cell to the next through the stack,electrically-conductive passages 234 are provided through the thicknessof a planar central portion or electrode-contacting zone 240 of the gasseparator plate corresponding in shape and size to the adjacentelectrode. The passages 234 comprise substantially perpendicularperforations through the plate 212 containing a silver or silver basedmaterial as described with reference to FIGS. 1 to 9. Although thepassages 234 through the gas separator plate 212 are illustrated asvisible, they would be covered with an electrically-conductive coatingacross the central portion 236 on each side, also as previouslydescribed herein.

Whilst the above description includes the preferred embodiments of theinvention, it is to be understood that many variations, alterations,modifications and/or additions may be introduced into the constructionsand arrangements of parts previously described without departing fromthe essential features or the spirit or ambit of the invention.

Other aspects of the gas separators described herein and their uses aredisclosed and claimed in copending International patent applicationsfiled concurrently herewith entitled Solid Oxide Fuel Cell StackConfiguration and Seal for a Fuel Cell Stack, respectively claimingpriority from Australian provisional patent applications PR6364 andPR6366 filed 13 Jul. 2001, and the contents of both of said copendingInternational patent applications and of their US national phaseequivalents are incorporated herein by reference.

It will be also understood that where the word “comprise”, andvariations such as “comprises” and “comprising”, are used in thisspecification, unless the context requires otherwise such use isintended to imply the inclusion of a stated feature or features but isnot to be taken as excluding the presence of other feature or features.

The reference to any prior art in this specification is not, and shouldnot be taken as, an acknowledgment or any form of suggestion that suchprior art forms part of the common general knowledge.

1. A fuel cell gas separator for use between two solid oxide fuel cells,the gas separator having a separator body with an anode-facing side anda cathode-facing side and with paths of electrically conductive materialtherethrough from the anode-facing side to the cathode-facing side in anelectrode-contacting zone, wherein the electrically conductive materialforming at least part of the length of each path is a silver-glasscomposite.
 2. A gas separator according to claim 1 wherein the materialof the separator body is zirconia.
 3. A gas separator according to claim2 wherein the zirconia contains up to about 20 wt % alumina.
 4. A gasseparator according to any one of claims 1 to 3 wherein the silver-glasscomposite contains from about 10 to about 40 wt % glass.
 5. A gasseparator according to claim 4 wherein the silver-glass compositecontains from about 15 to about 30 wt % glass.
 6. A gas separatoraccording to any one of claims 1 to 5 wherein the silver in thesilver-glass composite is commercially pure silver.
 7. A gas separatoraccording to any one of claims 1 to 5 wherein the silver in thesilver-glass composite is a silver alloy or mixture.
 8. A gas separatoraccording to claim 7 wherein the silver is alloyed or mixed with any oneor more of gold, palladium, platinum and stainless steel.
 9. A gasseparator according to any one of claims 1 to 8 wherein the glass in thesilver-glass composite is stable against crystallisation.
 10. A gasseparator according to any one of claims 1 to 9 wherein the glass in thesilver-glass composite is a high silica glass.
 11. A gas separatoraccording to claim 10 wherein the composition of the glass is 0-5.5 wt %Na₂O, 8-14 wt % K₂O, 0-2.2 wt % MgO, 1-3 wt % CaO, 0-6 wt % SrO, 0-8 wt% BaO, 6-20 wt % B₂O₃, 3-7 wt % Al₂O₃, 58-76 wt % SiO₂ and 0-10 wt %ZrO₂.
 12. A gas separator according to claim 11 wherein the compositionof the glass is 0-2.0 wt % Na₂O, 8-13.5 wt % 120, 0-0.05 wt % MgO, 1-1.6wt % CaO, 0.5-1 wt % SrO, 0-4.4 wt % BaO, 6-20 wt % B₂O₃, 3-6.0 wt %Al₂O₃, 60-75 wt % SiO₂ and 0-5.0 wt % ZrO₂.
 13. A gas separatoraccording to any one of claims 1 to 12 wherein a respective electricallyconductive coating is provided on the silver-glass composite at theanode-facing side and at the cathode-facing side of the separator body.14. A gas separator according to claim 13 wherein each of said coatingsextends over the respective electrode-contacting zone.
 15. A gasseparator according to claim 13 or claim 14 wherein the coating on thecathode-facing side is of Ag or Ag alloy.
 16. A fuel cell gas separatorfor use between two solid oxide fuel cells, the gas separator having azirconia-based body with an anode-facing side and a cathode-facing sideand with paths of electrically conductive material therethrough from theanode-facing side to the cathode-facing side in an electrode-contactingzone of the separator body, wherein the electrically conductive materialforming at least part of the length of each path is silver or asilver-based material and wherein a coating of nickel on theelectrode-contacting zone on the anode-facing side overlies said silveror silver-based material in the paths of electrically conductivematerial and a coating of Ag or Ag—Sn alloy on the electrode contactingzone on the cathode-facing side overlies said silver or silver-basedmaterial in the paths of electrically conductive material.
 17. A gasseparator according to claim 16 wherein the zirconia of the separatorbody is yttria-stabilised.
 18. A gas separator according to claim 16wherein the zirconia of the separator body contains up to about 20 wt %alumina.
 19. A gas separator according to any one of claims 16 to 18wherein the silver or silver-based material is metallic silver.
 20. Agas separator according to any one of claims 16 to 18 wherein the silveror silver-based material is a silver alloy or mixture.
 21. A gasseparator according to claim 20 wherein the silver is alloyed or mixedwith any one or more of gold, palladium, platinum and stainless steel.22. A gas separator according to any one of claims 16 to 18 wherein thesilver or silver-based material is a silver-glass composite.
 23. A gasseparator according to any one of claims 16 to 22 wherein the coating onthe anode-facing side is of commercially pure nickel.
 24. A gasseparator according to any one of claims 16 to 23 wherein the layer ofnickel on the anode-facing side has a thickness in the range of 10 to1000 μm.
 25. A gas separator according to any one of claims 16 to 24wherein a layer of silver is disposed on the electrode-contacting zonebetween the coating of nickel and the anode-facing side of the gasseparator body.
 26. A gas separator according to claim 25 wherein thelayer of silver comprises commercially pure silver.
 27. A gas separatoraccording to claim 25 or claim 26 wherein the layer of silver has athickness in the range of 10 to 1000 μm.
 28. A gas separator accordingto any one of claims 15 to 27 wherein the coating on the cathode-facingside is Ag—Sn alloy that contains from about 4 to about 20 wt % Sn. 29.A gas separator according to any one of claims 15 to 28 wherein thecoating on the cathode-facing side is Ag—Sn alloy that includes up to 10wt % of dopants to improve the electrical conductivity of said coating.30. A gas separator according to any one of claims 15 to 29 wherein thecoating on the cathode-facing side is Ag—Sn alloy and has a thickness inthe range of 10 to 1000 μm.
 31. A gas separator according to any one ofclaims 15 to 30 wherein the coating on the cathode-facing side is Ag—Snalloy having a surface layer of SnO₂.
 32. A gas separator according toany one of claims 15 to 27 wherein the coating on the cathode-facingside is of commercially pure silver and has a thickness in the range of50 to 250 μm.
 33. A gas separator according to any one of claims 1 to 32wherein the paths of electrically conductive material are formed inperforations through the separator body.
 34. A gas separator accordingto claim 33 wherein the perforations extend perpendicularly through thethickness of the separator body.
 35. A gas separator according to claim34 wherein each path of electrically conductive material at the anodeside of the separator body is offset relative to a connected path ofelectrically conductive material at the cathode side.
 36. A gasseparator according to any one of claims 1 to 35 wherein each path ofelectrically conductive material has an average cross-sectionaldimension in the range of 50 to 1000 μm.
 37. A gas separator accordingto any one of claims 1 to 36 wherein the total area of the paths ofelectrically conductive material through the separator body is in therange of 0.1 to 20 mm² per 1000 mm² surface area (measured on oneside-only) of the electrode-contacting zone.
 38. A gas separatoraccording to any one of claims 1 to 37 wherein the separator body is inthe form of a plate.
 39. A gas separator according to any one of claims1 to 38 wherein surface formations defining gas flow passagestherebetween are provided on each of the anode-facing side andcathode-facing side in the electrode-contacting zone, said surfaceformations being electrically conductive and overlying the paths ofelectrically conductive material.
 40. A gas separator according to claim39 wherein the surface formations on the anode side are formed of solidoxide fuel cell anode material and the surface formations on the cathodeside are formed of solid oxide fuel cell cathode material, said surfaceformations being bonded to the separator body or to any coating in theelectrode-contacting zone.
 41. A gas separator according to claim 39 orclaim 40 wherein a respective electrically conductive coating isprovided over the surface formations on the anode-facing side and on thecathode-facing side.
 42. A gas separator according to claim 41 whereinthe coating on the surface formations on the cathode-facing side is ofmetallic silver.
 43. A gas separator according to claim 41 or claim 42wherein the coating on the surface formations on the anode-facing sideis of nickel.